Add IEmailService interface + StubEmailService

- IEmailService in Domain/Interfaces for membership notifications
- StubEmailService in Infrastructure/Services logs instead of sending
- TODO comments for future SMTP implementation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/Koogle.Domain/Interfaces/IEmailService.cs

@@ -0,0 +1,49 @@
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Koogle.Domain.Interfaces
+{
+    /// <summary>
+    /// Service interface for email notifications.
+    /// </summary>
+    public interface IEmailService
+    {
+        /// <summary>
+        /// Sends notification to club admins about a new membership request.
+        /// </summary>
+        /// <param name="clubId">The club receiving the membership request.</param>
+        /// <param name="userDisplayName">Display name of the requesting user.</param>
+        /// <param name="userEmail">Email of the requesting user.</param>
+        /// <param name="ct">Cancellation token.</param>
+        Task SendMembershipRequestNotificationAsync(
+            Guid clubId,
+            string userDisplayName,
+            string userEmail,
+            CancellationToken ct = default);
+
+        /// <summary>
+        /// Sends notification to user that their membership was approved.
+        /// </summary>
+        /// <param name="userEmail">Email of the approved user.</param>
+        /// <param name="clubName">Name of the club.</param>
+        /// <param name="ct">Cancellation token.</param>
+        Task SendMembershipApprovedAsync(
+            string userEmail,
+            string clubName,
+            CancellationToken ct = default);
+
+        /// <summary>
+        /// Sends notification to user that their membership was rejected.
+        /// </summary>
+        /// <param name="userEmail">Email of the rejected user.</param>
+        /// <param name="clubName">Name of the club.</param>
+        /// <param name="reason">Optional rejection reason.</param>
+        /// <param name="ct">Cancellation token.</param>
+        Task SendMembershipRejectedAsync(
+            string userEmail,
+            string clubName,
+            string? reason,
+            CancellationToken ct = default);
+    }
+}

src/Koogle.Infrastructure/DependencyInjection.cs

@@ -4,6 +4,7 @@ using Koogle.Infrastructure.Data;
 using Koogle.Infrastructure.Identity;
 using Koogle.Infrastructure.Repositories;
 using Koogle.Infrastructure.Security;
+using Koogle.Infrastructure.Services;
 using KoogleApp.Data;
 using Microsoft.AspNetCore.Authorization;
 using Microsoft.AspNetCore.Identity;
@@ -84,6 +85,9 @@ public static class DependencyInjection
         services.AddScoped<IDayRepository, DayRepository>();
         services.AddScoped<IPersonExpenseRepository, PersonExpenseRepository>();
 
+        // Services
+        services.AddScoped<IEmailService, StubEmailService>();
+
         services.AddCascadingAuthenticationState();
 
         return services;

src/Koogle.Infrastructure/Services/StubEmailService.cs

@@ -0,0 +1,80 @@
+using Koogle.Domain.Interfaces;
+using Microsoft.Extensions.Logging;
+using System;
+using System.Threading;
+using System.Threading.Tasks;
+
+namespace Koogle.Infrastructure.Services;
+
+/// <summary>
+/// Stub email service that logs instead of sending emails.
+/// TODO: Replace with actual SMTP implementation.
+/// </summary>
+public class StubEmailService : IEmailService
+{
+    private readonly ILogger<StubEmailService> _logger;
+
+    /// <summary>
+    /// Creates a new StubEmailService instance.
+    /// </summary>
+    public StubEmailService(ILogger<StubEmailService> logger)
+    {
+        _logger = logger;
+    }
+
+    /// <inheritdoc />
+    public Task SendMembershipRequestNotificationAsync(
+        Guid clubId,
+        string userDisplayName,
+        string userEmail,
+        CancellationToken ct = default)
+    {
+        _logger.LogInformation(
+            "[EMAIL STUB] Membership request notification - ClubId: {ClubId}, User: {UserDisplayName} ({UserEmail})",
+            clubId, userDisplayName, userEmail);
+
+        // TODO: Implement actual email sending via SMTP
+        // - Get club admin emails from database
+        // - Build notification template
+        // - Send via configured SMTP provider
+
+        return Task.CompletedTask;
+    }
+
+    /// <inheritdoc />
+    public Task SendMembershipApprovedAsync(
+        string userEmail,
+        string clubName,
+        CancellationToken ct = default)
+    {
+        _logger.LogInformation(
+            "[EMAIL STUB] Membership approved - To: {UserEmail}, Club: {ClubName}",
+            userEmail, clubName);
+
+        // TODO: Implement actual email sending via SMTP
+        // - Build approval notification template
+        // - Include link to dashboard
+        // - Send via configured SMTP provider
+
+        return Task.CompletedTask;
+    }
+
+    /// <inheritdoc />
+    public Task SendMembershipRejectedAsync(
+        string userEmail,
+        string clubName,
+        string? reason,
+        CancellationToken ct = default)
+    {
+        _logger.LogInformation(
+            "[EMAIL STUB] Membership rejected - To: {UserEmail}, Club: {ClubName}, Reason: {Reason}",
+            userEmail, clubName, reason ?? "(no reason provided)");
+
+        // TODO: Implement actual email sending via SMTP
+        // - Build rejection notification template
+        // - Include reason if provided
+        // - Send via configured SMTP provider
+
+        return Task.CompletedTask;
+    }
+}